The present invention relates generally to cardiac pacers, and more particularly to improvements in such pacers which provide the capability of determining the stimulation threshold level of a pacer-driven heart with minimal interruption to the normal pacing regimen.
Cardiac pacers, which supply amplitude and rate-controlled electrical pulses to a patient's heart to stimulate muscle contraction, have been developed for both internal applications, wherein the pacer is implanted within the patient's body, and external applications, wherein the pacer is externally carried by the patient and rate, pulse amplitude and sensitivity are adjusted as required by the application. External pacers are typically used in emergency situations, where the patient is waiting implant of a permanent pacer, or where the operation of other pacers is being tested or adjusted. These pacers are connected to the heart by pacer leads which extend into one or more chambers of the heart, and are designed to operate over a wide range of parameters to accommodate physiological changes in the heart and variations in the placement of the pacer leads.
One parameter of particular interest is the minimum current or stimulation threshold required to consistently cause rhythmic contraction of the heart. This stimulation threshold is primarily a function of the size and positioning of the pacer lead tip, and may change from an acute value following initial implant, to a chronic value several weeks or months after the implant, as a result of fibrin growth at the lead-tissue interface. An abnormally high threshold level may indicate that the leads are not making adequate contact with the heart tissue, and that a repositioning of the pacer lead must be accomplished if reliable pacing is to be obtained. Furthermore, knowledge of the threshold level assists the physician in determining the optimum output current level setting for the pacer.
In determining stimulation threshold with prior art external pacers it has been necessary for the operator to readjust the output level controls of the pacers to successively lower settings, observing as the threshold level the output current level at which the heart ceases to rhythmically contract. This has resulted in the normal operating-level stimulation pulse being removed from the heart for undesirably long periods of time while the output level control is repositioned to lower threshold levels through intervening intermediate levels. The present invention is directed to a pacer output circuit which avoids this drawback by enabling the operator to instantaneously switch to a known lower threshold current level from a known higher operating level and back, without having to operate at intermediate levels.
Accordingly, it is a general object of the present invention to provide a new and improved cardiac pacer.
It is a more specific object of the present invention to provide a new and improved cardiac pacer having means for quickly determining the threshold stimulation level of an associated heart.
It is another specific object of the present invention to provide a cardiac pacer having operator-actuated means for instantaneously reducing the current level of stimulation pulses produced by the pacer to enable the threshold level of an associated heart to be determined with minimal interruption to the normal pacing regimen.